1. Field of the Invention
This invention relates to chromatography. More particularly, this invention relates to a detector for monitoring the output of a chromatographic column to provide signals responsive to the concentration of components separated by the column.
Chromatography is an analytical procedure wherein the components of a mixture are separated so that the individual component concentrations can be determined. In operation, a sample of the mixture is conducted by a carrier fluid through a column containing a material which retains the mixture components for differing periods of time so that the components are physically separated to emerge at different times from the column. By providing a suitable detector at the output end of the column, measurement signals are developed responsive to component concentration. Such signals may be used to develop a so-called chromatogram comprising a series of time-separated signal peaks each having a height corresponding to the concentration of a respective component.
This invention relates to an improved chromatographic detector, and particularly to detection means suitable for use in a so-called process chromatograph. A process chromatograph is one which is utilized directly with an industrial process to continuously monitor the concentration of a limited number of components (frequently just one). Such chromatograph operates continuously to analyze a series of sequential samples to develop a corresponding series of signals indicating the concentration of the component(s) of interest. The resulting output of the chromatograph over a number of such analysis cycles defines one or more so-called "trend" signals showing the change in concentration of the component(s) of interest with respect to time.
2. Description of the Prior Art
Chromatography has been used extensively for a number of years for component concentration analysis, and a wide variety of different types of detectors have been proposed for producing signals responsive to concentration of the separated components. Some of these detectors have gone into extensive commercial use for laboratory type analyses, particularly the thermal-conductivity cell and the flame-ionization detector. Use of such commercially available prior art detectors for process chromatography has however posed a number of problems including cost, inadequate reliability, and potential danger to the process itself.
Other types of detectors have been suggested from time to time, but have not been found to be satisfactory. For example, U.S. Pat. No. 3,354,696, issued Nov. 28, 1967, teaches the use of means responsive to pressure drops developed by a bridge network of pneumatic resistors connected to the output of the column. The pneumatic resistors may either be capillaries, which are used to detect changes in gas viscosity, or screens, which detect changes in gas density. Another somewhat similar detector arrangement, using a pair of capillaries to develop pressure drops, is shown in an article published in Transactions of the Faraday Society, line 63, number 8, pages 1895- 1905, 1967.
A major problem associated with proposed pneumatic detectors of the type referred to in the preceding paragraph is that the pressure signal produced by a pneumatic resistor is highly responsive to the rate of fluid flow through the resistor. Thus, variations in flow velocity through the chromatographic column cause changes in the effective base line of the measurement signal, tending to cause errors in the final measurement.
It has been proposed that such errors due to changes in column flow rate be avoided by carefully regulating the pressure or flow rate at the input of the column. However, for reasons primarily related to complex column dynamics, such pressure or flow regulation has not satisfactorily solved the problem.
It also has been proposed that errors due to changes in column flow rate be compensated for, i.e. nullified, by providing a second column and detector in parallel with the primary column and detector, and connecting the two detector outputs in series opposition. By injecting the sample only into the primary column, there will be no component measurement signals developed in the secondary detector, and thus the combined primary and secondary detector signals will reflect the desired concentration measurements. If there is a change in flow rate through both columns, e.g. due to a change in the pressure of the carrier entering the columns, there will presumably be corresponding and equal flow-responsive variations in the detector output signals. Since the detector outputs are connected in series-opposition, the flow-responsive variations in the primary detector output should be nullified by the equal and opposite variations in the secondary detector output, thus leaving the component measurement signals unaffected by flow rate.
Although such a dual-column compensation system apparently would be satisfactory if the flow rate changes in both columns were always equal, there are practical operating conditions under which such equality will not be maintained. For example, when a sample is injected into a column, it causes a change in the flow rate within that column, particularly when the sample contains components having viscosities quite different from that of the carrier, and thereby at once alters the base line of the detector signal. As each separated component emerges from the column, and thus is no longer present in the column to affect the flow rate, there is a corresponding step-change in the flow rate of fluid past the detector, with a consequent step-change in the detector signal base line. Because no sample is injected in the secondary column, such changes in the primary detector signal are not duplicated in the secondary detector, and therefore the secondary column and detector cannot compensate for the errors in the primary detector signal. Consequently, the dual-column compensation approach has not provided a satisfactory solution to the problem.